Wearable exoskeletons have been designed for medical, commercial, and military applications. Medical exoskeleton devices are being developed to restore and rehabilitate proper muscle function for people with disorders that affect muscle control. Medical exoskeleton devices are a system of motorized braces that can apply forces to the wearer's appendages. In a rehabilitation setting, medical exoskeletons are controlled by a physical therapist who uses one of a plurality of possible input means to command an exoskeleton control system. In turn, the medical exoskeleton control system actuates the position of the motorized braces, resulting in the application of force to, and typically movement of, the body of the exoskeleton wearer. Medical exoskeletons can also be used outside of a therapeutic setting to grant improved mobility to a disabled individual. Commercial and military exoskeletons help prevent injury and augment the exoskeleton wearer's strength. Commercial and military exoskeletons are used to alleviate loads supported by workers or soldiers during their labor or other activities, thereby preventing worker injuries and increasing their stamina and strength. Tool-holding exoskeletons are outfitted with a tool-holding arm that supports the weight of the tool, reducing user fatigue by providing tool-holding assistance. The tool-holding arm transfers the vertical force required to hold the tool through the legs of the exoskeleton rather than through the user's arms and body. Similarly, weight-bearing exoskeletons transfer the weight of the exoskeleton load through the legs of the exoskeleton rather than through the user's legs. In some cases, weight-bearing exoskeletons are designed to carry a specific load, such as a heavy backpack. In other cases, military weight-bearing exoskeletons support the weight of armor. Commercial and military exoskeletons can have actuated joints that augment the strength of the exoskeleton wearer, with these actuated joints being controlled by the exoskeleton control system, and with the exoskeleton wearer using any of a plurality of possible input means to command the exoskeleton control system.
In powered exoskeletons, exoskeleton control systems prescribe and control trajectories in the joints of an exoskeleton, resulting in the movement of the exoskeleton. These control trajectories can be prescribed as position-based, force-based, or a combination of both methodologies, such as those seen in impedance controllers. Position-based control systems can be modified directly through modification of the prescribed positions. Force-based control systems can also be modified directly through modification of the prescribed force profiles. Complicated exoskeleton movements, such as walking in an ambulatory medical exoskeleton, are commanded by an exoskeleton control system through the use of a series of exoskeleton trajectories, with increasingly complicated exoskeleton movements requiring an increasingly complicated series of exoskeleton trajectories. These series of trajectories can be cyclic, such as the exoskeleton taking a series of steps with each leg, or they can be discrete, such as an exoskeleton rising from a seated position into a standing position. In the case of an ambulatory exoskeleton, during a rehabilitation session and/or over the course of rehabilitation, it is highly beneficial for the physical therapist to have the ability to modify the prescribed positions and/or the prescribed force profiles depending on the particular physiology or rehabilitation stage of a patient. As various exoskeleton wearers may be differently proportioned, variously adjusted or customized powered exoskeletons will fit each wearer somewhat differently. The exoskeleton control system should take into account these differences in wearer proportion, exoskeleton configuration/customization, and exoskeleton-wearer fit, resulting in changes to prescribed exoskeleton trajectories.
While exoskeleton control systems assign trajectories to the joints of the exoskeleton and control the positions of these joints, the actual forces applied to exoskeleton joints are exerted by actuators. These actuators can take many forms, as is known in the art, each with advantages and disadvantages in various applications. In current exoskeletons, the actuator exerting force on a joint is typically constituted by an electric motor located proximal to that joint. Co-location of the actuator with the joint has advantages in terms of mechanical and design simplicity, but it has certain disadvantages—foremost among them is that adding an electric motor to a joint increases the bulk of the joint, limiting maneuverability of the joint and exoskeleton in certain environments. In comparison, consider a human finger: the musculature exerting force on the joints of the finger is not located near the joints of the finger but rather in the forearm, with muscular contraction pulling on tendons that relay that force over distance to the joints of the finger. This has the advantage of minimizing the bulk of the fingers, allowing for both greater dexterity and closer placement of the fingers to each other. In addition, more muscle can be located in the arm than would fit on the fingers, allowing for greater strength. One mechanical actuation device, described in U.S. Pat. No. 4,843,921, uses a drive mechanism in which an electric motor twists on a loop of cord, with this cord loop forming a helical structure and shortening as it is twisted, causing the length of cord to shorten and pull the two ends of the cord loop closer together. In this way, the activation of the electric motor is used to apply a pulling force over distance through the cord loop. This allows for a design in which the motor driving the movement of a joint is located at a position distal from the joint.
In biological joints, muscles exert force by shortening their length, resulting in translation of the tensile member (tendon) exerting force over distance. This differs from the twisting tensile members as seen in motor-and-loop actuators such as that shown in U.S. Pat. No. 4,843,921. One disadvantage of using such actuators for larger joints in the human exoskeleton, such as the knee or hip, is that even high tensile strength cord, after being twisted and untwisted many times, or having been subject to stress from a fall or misstep, may be at risk of breakage, with such a failure resulting in uncontrolled joint movement. Further, regulatory bodies, such as the Food and Drug Administration, require that medical devices comply with safety guidelines—without addressing the risks of such a design, such an exoskeleton actuator design is unlikely to be approved for use by these agencies. In military exoskeletons, the failure of a joint may not result in direct injury to the wearer, but any reduced mobility could be dire in a combat situation.
A ball screw is a mechanical device that allows rotational motion to be translated into linear motion. These devices, such as those described in U.S. Pat. Nos. 2,855,791 and 3,667,311, or other forms known in the art, are comprised of a threaded shaft that acts as a raceway for ball bearings and a nut that moves over the ball bearings as it travels along the threaded shaft, with the translation of the ball nut along the length of the shaft being driven by the rotation of the threaded shaft. These devices have mechanisms to allow the recirculation of ball bearings within the ball nut as the ball nut moves along the threaded shaft. Ball screw devices are used in automotive steering, aircraft or missile control surfaces, and robotics systems—including high-precision robotic systems such as those used in semiconductor manufacturing. Ball screws are designed for straight-line axial thrust, with any type of transverse force or side load greatly reducing ball screw life and rapidly decreasing mechanical efficacy prior to failure. To prevent side loads, ball screw actuators include devices such as precision rails and/or linear bearings, or other similar devices known in the art. The devices to prevent side load tend to be substantial in relation to the ball screw, increasing the size and weight of an actuator system (both undesirable characteristics in exoskeleton applications).
It is conceivable that an exoskeleton actuator could be developed that combines the linear motion (and high efficiency) of a ball screw system with the force-transfer-over-distance features of tensile member actuators. Such a system would allow the electric motors and other components of the ball screw to be placed away from the joint, and the linear motion of the ball screw would allow the tensile members to transfer force by translating rather than twisting, resulting in decreased wear. However, a number of disadvantages to such a design exist as well. First, the mechanisms of most robotic/exoskeleton actuators allow the actuator to exert force in two joint movement directions, those being flexion and extension in the case of the knee. However, a tensile member is only able to transfer force through a pulling motion, allowing force to be applied to a joint and effecting motion in only one direction. While some exoskeleton joint actuators have used springs or other similar devices to cause a joint to return to a position when the current to the motor is disengaged, this is not suitable for the forces required to move the large joints of a human exoskeleton and/or the body of the wearer.
There exists an unmet need for a device for use in human exoskeletons that allows for force to be exerted on a joint, effecting bidirectional movement of the joint, with this device being located away from the joint. There further exists a need for this device to be highly efficient at the transfer of force from an electrical motor or other power source to the joint, minimizing energy consumption and/or maximizing force applied to the joint. There further exists a need for this device to be low profile and add little bulk at the joint being powered. There further exists a need for this device to incorporate a robust and simple system for force sensing, allowing the exoskeleton control system to accurately control the position and force applied to the joint.